1. Field of the Invention
The present invention relates to a receiver for receiving digital audio broadcasting signals, and more particularly to a receiver equipped with a demodulator for demodulating a signal modulated through orthogonal frequency division multiplex (hereinafter described also as OFDM).
2. Description of the Related Art
EP-A-0 606941 and M. Lusie et al.: xe2x80x9cCarrier Frequency Acquisition and Tracking for OFDM Systemsxe2x80x9d, IEEE Translation on Communication, Vol.44 No.11, November 1996 disclose a conventional demodulator for demodulating a signal modulated through OFDM. In such conventional demodulator shown in FIG. 2, an OFDM-modulated reception signal is supplied to an input terminal, frequency-converted by a mixer 1, and A/D converted by an A/D converter 2. The A/D converted reception signal is converted into I/Q data of a baseband by an I/Q detector (orthogonal detector) 3, which I/Q data is supplied to a fast Fourier transform circuit (hereinafter described as an FFT circuit) 4. The FFT circuit 4 Fourier-transforms and converts the input time domain signal into a frequency domain signal. An output from the FFT circuit 4 is supplied to a delay/detection unit 6 and to a frequency offset detector 40. The delay/detection unit 6 detects the input signal through one-symbol delay and outputs the demodulated data via an output terminal.
The frequency offset detector 40 detects a frequency offset amount in accordance with a phase reference symbol contained in the input signal. The detected frequency offset amount is D/A converted by a D/A converter 41 and supplied to a voltage controlled oscillator 9 which functions as a local oscillator. In accordance with the detected and D/A converted frequency offset amount, the oscillation frequency of the voltage controlled oscillator 9 is controlled to correct a frequency shift, so that the output of the voltage controlled oscillator 9 with the frequency shift being corrected, is supplied to the mixer 1 for the frequency conversion.
The conventional receiver described above is, however, associated with some problem that normal signal reception may become unable under multi-path reception environments, because of a large amount of a detected frequency offset.
It is an object of the present invention to provide a receiver capable of normal signal reception even under severe reception environments such as multi-path reception environments.
According to one aspect of the present invention, a receiver with a demodulator which A/D converts a reception signal, discrete-Fourier-transforms the A/D converted signal, and delay-detects the discrete-Fourier-transformed data, is provided which comprises: first frequency offset detecting means for detecting a frequency offset value of an input signal to the demodulator in accordance with a phase reference symbol contained in the discrete-Fourier-transformed data; second frequency offset detecting means for detecting a frequency offset value of the input signal to the demodulator in accordance with a phase difference of the delay-detected signal; an integrator for integrating the frequency offset value detected by the first frequency offset detecting means; and selecting means for comparing an output of the integrator with a predetermined first range of threshold values, selecting the frequency offset value detected by the second frequency offset detecting means if the output of the integrator is in the first threshold value range, and selecting the frequency offset value detected by the first frequency offset detecting means if the output of the integrator is not in the first threshold value range, wherein automatic frequency control is performed in accordance with the frequency offset value selected by the selecting means.
According to the receiver of this invention, the frequency offset value detected by the first frequency offset detecting means and integrated by the integrator is compared with the first threshold value range. If an output of the integrator is in the first threshold value range, the frequency offset value detected by the second frequency offset detecting means is selected, whereas if the output of the integrator is not in the first threshold value range, the frequency offset value detected by the first frequency offset detecting means is selected. In accordance with each selected frequency offset value, the automatic frequency control is performed.
The receiver of this invention further comprises: comparing means for comparing a predetermined second range of threshold values with the frequency offset value detected by the first frequency offset detecting means; and loading means for loading a predetermined value in the integrator if it is judged from a comparison by the comparing means that the frequency offset value detected by the first frequency offset detecting means is in the second threshold value range and if the selecting means selects the frequency offset value detected by the first frequency offset detecting means.
In the case of the receiver of the invention provided with the loading means, if the frequency offset value detected by the first frequency offset detecting means is in the second threshold value range and the frequency offset value detected by the first frequency detecting means is selected for automatic frequency control, then the frequency offset value detected by the first frequency detecting means can be switched at once to the frequency offset value detected by the second frequency detecting means. Therefore, frequent switching can be avoided which may otherwise be caused by external disturbances.
The receiver of this invention further comprises: comparing means for comparing a predetermined third range of threshold values narrower than the second threshold value range, with the frequency offset value detected by the first frequency offset detecting means; and clearing means for clearing the integrator if it is judged from a comparison by the comparing means that the frequency offset value detected by the first frequency offset detecting means continues in the second threshold value range during a predetermined period.
In the case of the receiver of the invention provided with the clearing means, automatic frequency control can be maintained by the frequency offset value detected by the second frequency offset detecting means, even if instantaneous noises are generated. Therefore, the automatic frequency control will not be switched by instantaneous noises to the frequency offset value detected by the first frequency offset detecting means.